Articles made from mixtures of cement and asbestos containing from about 10% to 50% by weight of asbestos fibers are employed today in large quantities. The manufacture and sale of these asbestos-cement articles are commercially very important. Sheets and boards of the asbestos-cement type find widespread use in the building and construction industries because they are fireproof and have other desirable qualities.
Asbestos-cement products contain a hydraulic cement which has to be "set" or "cured" to achieve the final characteristic high strength of these products. Curing of asbestos-cement products may be accomplished under any conditions known to be useful for setting the cement to form final products of high strength. One curing method used where portland cement is employed is to store the final structure at room temperatures under highly humid conditions for about 7 to 30 days.
It is often desirable and necessary, however, to employ a shorter curing time. One method of achieving shorter curing times is to "autoclave" cure the asbestos-cement product in an autoclave under pressure of saturated steam for about 12 to 20 hours. Normally, an auxiliary cementing agent such as silica flour or quartz is used as a component of the cement binder if the final product is to be of steam cured, The silica material, under the conditions which prevail in an autoclave, reacts with the calcium hydroxide from the hydrated hydraulic cement to form part of the final binding matrix. Autoclave curing because of its rapidity is employed advantageously in commercial production of asbestos-cement products.
In the past, asbestos-cement products that have been steam or "autoclave" cured in an autoclave have provided thermal shock resistance properties at least equal to that of competitive building materials. These autoclaved asbestos-cement products have, however, tended to flake, exhibit large volume changes, or even explode when they are subjected to high temperatures when wet or saturated with water. An improvement in this aspect of the already high thermal shock resistance properties of asbestos-cement products is desirable because of the possibility of the asbestos-cement product becoming saturated with water during fire-fighting operations and then being exposed to intense heat.